1. Field of the Invention
The present invention relates to electronic relay switches. More particularly, the present invention relates to a means for both decreasing the amount of heat generated by such switches and improving the ability of electronic relay switches to transfer heat away from the switching element.
2. Brief Description of the Prior Art
Presently available electronic relay switching devices include those composed of two refractory metal rods encased in an evacuated capsule. The two rods are commonly at right angles to each other, and each is electrically connected to one or more contact tabs which extend from the capsule. One of the rods is fixed, the other is movable through or pivots about a thin diaphragm located at one end of the capsule through which the pivotal, or movable, rod extends. In certain embodiments, a ceramic tube is attached to an extension of the movable rod which protrudes through the diaphragm. Electromagnetic or mechanical means are normally employed to pivot the ceramic tube and the movable rod in the appropriate direction towards and at right angles to the fixed rod until they touch, thus activating the relay switch. Angular motion is transmitted to the movable rod through the diaphragm so that the distance between the two crossed rods (fixed and movable) can be changed from zero when touching to a predetermined distance which is selected to accommodate the dielectric withstand voltage. Thus the distance that the movable rod must pivot is a small amount, commonly varying from 0.001 to 0.020 inches.
In the closed or activated position of the switch described, current flows through a contact tab extending out from the body of the switch, which tab may be adjacent the diaphragm. Current then flows through the diaphragm, and into and along the movable rod until it meets the contact point on the fixed rod. The current then enters the fixed rod, and travels into its support structure and out of the switch through a second contact tab which is electrically connected to the fixed rod. In the open position, the rods are separated by the small gap described above, which due to the very high dielectric strength of the vacuum is capable of operating at voltage differentials in the range of up to 1000 to 2000 volts per 0.001 inch.
Typically, the position of the switch is controlled by an actuator contacting the ceramic tube attached to the movable rod. The actuator is attached to a circular flange located at the diaphragm end of the switch capsule. One example of an actuator and switch of the type described above can be seen in applicant's issued U.S. Pat. No. 4,105,982, the disclosure of which is incorporated by reference, and which is hereinafter referred to as "my earlier Patent."
To transfer motion through the flexible diaphragm without requiring an excessive force to activate the relay switch, and to obtain a usable mechanical life of the diaphragm element for the motion necessary to obtain the decided upon open switch gap, it is necessary to use a very thin diaphragm element, sometimes on the order of 0.001 inches thick. This necessary thinness of the diaphragm element limits its ability to transfer heat away from the movable contact rod to the body and contact tabs of the switch.
It is also necessary to use a movable rod having as small a diameter as possible to obtain an acceptable operating and mechanical life of the diaphragm. This limits the ability of the rod to carry heat away from both the contact point of the two refactory rods and its internally generated losses to the diaphragm.
In order to build a switch with acceptable yields and still meet various operational requirements, it has been found that molybdenum is the preferred material for the diaphragm. However, molybdenum's electrical resistivity is considerably greater than that of copper. This results in generating a greater heat loss for a given current than copper. Also, the thermal conductivity of molybdenum is low compared to that of copper, thus realizing lesser heat transfer through the diaphragm than would be realized with a material such as copper.
The effect of the inadequate capability to dissipate heat, of the switches described above, is made even worse so when RF current travels through the switch because of "skin effect". As RF frequency increases, the effective depth of penetration of the current, from the surface of the conductor to the interior of the conductor, is decreased. In effect, the current is crowded towards the surface of the conductor. This in turn causes an increase in effective resistance which increases the heat generated by the current carrying elements.
In the case of the switch in question, the increase in RF current heating due to "skin effect" coupled with the limited thermal conductivity of the diaphragm and movable rod results in a sharp decrease in RF current carrying ability as frequency increases.